This invention relates generally to hydraulic devices such as an axle driving apparatus and, more particularly, to a control mechanism adapted to be mounted to the casing of a transaxle. The control mechanism disclosed herein could also be used with hydraulic pumps, hydrostatic transmissions, or other types of hydrostatic transaxles.
Hydrostatic transmissions (xe2x80x9cHSTsxe2x80x9d) are known in the art. Generally, an HST includes a center section on which is mounted a rotating hydraulic pump and a rotating hydraulic motor. The hydraulic pump and the hydraulic motor each carry a plurality of reciprocating pistons which are in fluid communication through hydraulic porting formed in the center section. Rotation of the hydraulic pump against a moveable swash plate creates an axial motion of the pump pistons that forces an operating oil through the hydraulic porting to the hydraulic motor to move the motor pistons. The axial motion of the motor pistons causes the hydraulic motor to rotate as the motor pistons bear against a thrust bearing. The hydraulic motor output may be used to drive the vehicle axles of a riding lawn mower, small tractor and the like. It is also known to use a hydraulic pump in its own housing, where the connection to a hydraulic motor is made through hoses or other connectors. An integrated hydrostatic transaxle (xe2x80x9cIHTxe2x80x9d) incorporates the elements of an HST in the same housing with output gearing, axles and the like.
In these applications, the position of the swash plate with respect to the hydraulic pump pistons is changed to adjust the speed and direction of rotation of the hydraulic motor and, accordingly, the output thereof. The orientation with which the swash plate addresses the hydraulic pump pistons can be changed to control whether the hydraulic motor rotates in the forward direction or in the reverse direction, and the speed at which the hydraulic motor rotates. There is also a neutral position in which the pump will displace insufficient hydraulic fluid to cause motor rotation.
It is known to use a trunnion arm coupled to the swash plate in order to change the swash plate orientation. A speed change lever or pedal is, in turn, coupled to the trunnion arm through a wire or other driving link. In this manner, movement of the speed change lever/pedal results in movement of the trunnion arm to change the position of the swash plate to thereby control the speed and direction of the vehicle. Examples of such mechanisms for adjusting the speed of a vehicle may be seen in U.S. Pat. No. 6,253,637, which is incorporated herein by reference in its entirety.
The present invention discloses an improved control mechanism comprising a return to neutral (xe2x80x9cRTNxe2x80x9d) mechanism and a cruise control mechanism that is adapted to be mounted to the casing of a hydraulic pump or hydrostatic transaxle. For simplicity of explanation the invention will be described in association with an integrated hydrostatic transaxle (xe2x80x9cIHTxe2x80x9d).
A rotatable trunnion arm is coupled to the moveable awash plate and extends from the casing to engage the control mechanism. A return to neutral mechanism acts to force the trunnion arm from the stroked position back to the neutral position. It will be understood that the position referred to herein as xe2x80x9cneutralxe2x80x9d need not be a perfect neutral; in some applications there may still be some hydraulic flow when the unit is in the chosen xe2x80x9cneutralxe2x80x9d position The amount of such flow permitted in this neutral position will depend upon the application and the design parameters.
The return to neutral feature is preferably unidirectional, in that it provides a return force when the unit is stroked in one direction, either reverse or forward, but does not provide any return force when the unit is stroked in the opposite direction. By way of example, in certain vehicle applications the user may want to have such a return force only when the vehicle is in reverse, but not in the forward direction, since the maintenance of the force needed to overcome the return force may be tiring to the user, or may be otherwise unnecessary. The cruise control feature preferably operates in the opposite direction so that, in this example, the cruise feature would be used when the vehicle is moving forward but not in reverse.
A better understanding of the objects, advantages, features, properties and relationships of the invention will be obtained from the following detailed description and accompanying drawings which set forth an illustrative embodiment and which are indicative of the various ways in which the principles of the invention may be employed.